The present invention relates generally to cold deformable, hot rolled steel bars and more particularly to high strength, hot rolled steel bars which can be cold deformed to a finished product having desirable physical properties without the need to employ heat treating either before or after the cold deforming operation.
Hot rolled steel bars are conventionally subjected to cold deforming operations such as cold drawing, cold extruding, cold heading and the like. An example of a finished product which has been subjected to cold deforming is a threaded fastener, such as a bolt or a screw. There are high-speed, fastener-making machines which form the fastener head, by cold heading, and roll the fastener threads, by cold rolling, in a single, continuous operation.
Hot rolled steel bars, from which threaded fasteners are made, must have sufficient ductility to enable one to readily perform the cold deforming operations. A conventional procedure for providing the desired ductility is to subject the hot rolled steel bar to a spheroidizing anneal, before cold deforming. The spheroidizing anneal causes practically all the carbides in the steel to agglomerate into globules or spheroids larger than one micron in diameter.
The finished, threaded fastener resulting from the cold deforming operations, should have good strength characteristics. A conventional procedure for providing the desired strength is to subject the finished, cold deformed product to a heat treatment which involved heating, quenching and tempering. Although this procedure increases the strength of the finished cold deformed product, it has other drawbacks. Heating, quenching and tempering can distort the product subjected to that treatment, so that a straightening operation would be required thereafter. Moreover, heat treating the threaded fastener after cold deforming removes residual compressive stress imparted to the threaded fastener by cold rolling the threads into the fastener, and that removal is undesirable.